1. Field of the Invention
The invention concerns an intruder detector, particularly one adapted to be mounted in the wall of a home or business, the detector incorporating an improved and simplified system for sensing and signalling the presence of an intruder within the areas viewed by the system.
2. Prior Art
Numerous systems have been designed and are presently in use to sense and signal the presence of an intruder within a defined area.
Some intrusion detection systems are active in nature, producing a signal within or confined to a secure area. The signal is affected by the presence of an intruder and this effect is employed to actuate an alarm or other signal. Not only do such active systems require a continuous energy input, but they are prone to false alarms and their presence can be detected by the intruder, all of which cause the system to be ignored, avoided or otherwise negated.
Some intrusion detection systems are of a passive nature. Most passive systems detect the presence of an intruder in a defined area by sensing and responding to visible or non-visible radiation of the intruder. Rossin U.S. Pat. No. 3,839,640, dated Oct. 1, 1974, presents an example of an intruder detection system responsive to infrared radiation, a system that is remarkably free from false alarms.
Intrusion detection systems are subjected to a number of non-intruder related inputs, all of which must be distinguished from an intruder related input if the system is to prove practical in use. Inputs which can cause passive infrared systems to produce false alarms include reflected sunlight from an airplane or automobile, or light being directed onto the detector from automobile headlights or a flashlight out of the detection area. An intermittent electric heater, or a flickering light bulb in the detector's field of view, also can actuate present systems, causing a false alarm.